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<title>Macro BOOST_PROTO_DEFINE_ENV_VAR</title>
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<h2><span class="refentrytitle">Macro BOOST_PROTO_DEFINE_ENV_VAR</span></h2>
<p>BOOST_PROTO_DEFINE_ENV_VAR — 
      Define a type and a global variable of that type that can be used
      to initialize a slot in a Proto transform environment.
    </p>
</div>
<h2 class="refsynopsisdiv-title">Synopsis</h2>
<div class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="proto/reference.html#header.boost.proto.transform.env_hpp" title="Header &lt;boost/proto/transform/env.hpp&gt;">boost/proto/transform/env.hpp</a>&gt;

</span>BOOST_PROTO_DEFINE_ENV_VAR(Type, Name)</pre></div>
<div class="refsect1">
<a name="id-1.3.31.5.46.22.4"></a><h2>Description</h2>
<p>
        Proto primitive transforms can optionally accept an environment in
        their third parameter which is a key/value store of environment
        variables. Use the <code class="computeroutput">BOOST_PROTO_DEFINE_ENV_VAR()</code> macro
        to define the keys.
      </p>
<p>
        See the description for <code class="computeroutput"><a class="link" href="boost/proto/data_type.html" title="Struct data_type">proto::data_type</a></code>
        for an example of the class interface created by this macro.
      </p>
<p>
        <span class="bold"><strong>Example:</strong></span>
      </p>
<p>
        </p>
<pre class="programlisting"><span class="identifier">BOOST_PROTO_DEFINE_ENV_VAR</span><span class="special">(</span><span class="identifier">mykey_type</span><span class="special">,</span> <span class="identifier">mykey</span><span class="special">)</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">FetchMyKey</span>
  <span class="special">:</span> <a class="link" href="boost/proto/when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span> <a class="link" href="boost/proto/_.html" title="Struct _">_</a><span class="special">,</span> <a class="link" href="boost/proto/_env_var.html" title="Struct template _env_var">proto::_env_var</a><span class="special">&lt;</span><span class="identifier">mykey_type</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span>
<span class="special">{</span>
    <a class="link" href="boost/proto/terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">}</span><span class="special">;</span>
    <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">sz</span> <span class="special">=</span> <span class="identifier">FetchMyKey</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="special">(</span><span class="identifier">mykey</span> <span class="special">=</span> <span class="string">"hello!"</span><span class="special">)</span><span class="special">)</span><span class="special">;</span>
    <span class="identifier">assert</span><span class="special">(</span><span class="number">0</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">strcmp</span><span class="special">(</span><span class="identifier">sz</span><span class="special">,</span> <span class="string">"hello!"</span><span class="special">)</span><span class="special">;</span>
<span class="special">}</span></pre>
<p>
      </p>
</div>
</div>
<div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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